In electronics, a DC-DC converter is an electronic circuit adapted to convert a direct current source from one voltage value to another voltage value.
One of the most relevant among the known types of DC-DC converters is represented by the switching DC-DC converters. Considering in particular a DC-DC converter of the “step-down” type, wherein the input voltage is converted into an output voltage having a lower value, an example of the operation thereof will be now described. During a first phase—denoted as “main phase”—the switching DC-DC converter electrically couples a source of the input voltage (to be converted) to a terminal of a reactive element, typically an inductor. As a consequence, in this phase the reactive element stores magnetic energy, absorbing an input current from the input voltage source. In this phase, the output load requiring the (converted) output voltage receives energy from the input voltage source. In a second phase—denoted “secondary phase”—the reactive element is disconnected from the input voltage source, and the output load receives the magnetic energy that has been stored in the reactive element during the main phase. Acting on the ratio between the main phase and secondary phase durations—denoted “duty cycle”—it is possible to regulate the transfer of energy from the voltage source to the output load in a controlled way, and thus bring the voltage provided to the output load to the requested value.
An important application field of the step down DC—DC converters regards the supplying of Central Processor Units (CPU) and Graphic Processor Units (GPU).
Modern processors are typically supplied with a switching DC-DC converter having a buck topology. Depending on the power consumption requirements, synchronous multiphase topologies may be employed. A multiphase topology is realized arranging N-single synchronous phase buck converters in parallel and controlling them with a same system control loop. The number of phase depends on the maximum current that the processor may drain, such as, two phases for maximum currents up to 40 A and eight phases for maximum currents up to 180 A.
Modern processors often require supply voltages having very stable values. For example, the maximum allowed deviation—caused by static accuracy, output voltage ripple, and drop due to heavy dynamic current transitions—may be equal to +/−5% (that is, for a supply voltage of 1 Volt, only +/−50 mV pp). The CPUs and GPUs of the latest generations have greatly increased their computational power; the current consumption (i.e., the amount of current drained in operation) of modern processors has changed from a “static” fashion to a “dynamic” fashion. More specifically, the current drained by modern processors may vary by a great extent based on the status of operation thereof with a high frequency (e.g., every microsecond). From an electrical point of view, modern processors may be modeled as current-pulse generators draining a current pulse train having a frequency repetition (Fl) up to 1 MHz with a high slew rate (e.g., up to 1000 A/μs).
The switching frequency (Fsw) of a standard switching DC-DC converter having the buck topology adapted to be exploited for supplying a processor is typically set (basically to increase the system efficiency) to a value ranging from approximately one to few hundreds of kHz.
The interference between the frequency repetition Fl of the current pulses drained by the load (the processor) and the switching frequency Fsw of the DC-DC converter may cause the generation of spurious beat oscillations (or frequency beats) at the (beat) frequencies of Fl−Fsw and Fl+Fsw (and multiples thereof). This phenomenon may negatively affect the correct operation of the converter, by, for example, introducing, in particular, low-frequency (Fl−Fsw) spurious oscillations in the current flowing through the reactive element (inductor) and in the output voltage generated by the converter. If sufficiently large, such spurious oscillations may cause the power components of the converters (such as the inductor and the switching elements) to be subjected to excessive electrical and thermal stresses and/or the value of the output voltage to exceed the maximum allowed deviation from the ideal value.
An example of such spurious beat oscillations is illustrated in the timing diagrams of FIG. 1, showing the simulated time course of the current flowing in the inductor (top) and of the output voltage (bottom) of an exemplary switching DC-DC buck converter operating at a switching frequency Fsw of 300 KHz for supplying a load draining a current pulse train having a frequency repetition Fl of 310 KHz (i.e., with a beat frequency of Fl−Fsw=10 kHZ).
In order to solve these and possibly other drawbacks, a first solution known in the art provides for using switching control units (i.e., the circuit that control the opening/closing of the switching elements of the converter) that exploit proper (typically, non-linear) techniques capable of eliminating the beat oscillations. However, switching control units of this type may be very complex and thus their cost may not be negligible.
A further known solution provides instead for using standard switching control units, but, at the same time, greatly reducing the bandwidth of the system for filtering out the beat oscillations. This solution may be very expensive, too. Indeed, a great bandwidth reduction slows the response time of the system; in order to restore the response time to comply with the output requirements, the output capacity needs to be increased, increasing thus the manufacturing cost.